The Diversification of Morphology

09/26/2017

Transcriptomic exploration
of the coleopteran wings reveals insight into the mechanisms underlying
evolution of the novel elytron structure

The acquisition of morphologically novel structures and their subsequent diversification is central to evolution, yet the molecular mechanisms that underlie this
process are still elusive. The unique forewings of beetles (called elytra) can
serve as a powerful model to study these mechanisms. In the past, the orthologs
of genes important for Drosophila
wing formation served as the starting point for studies examining the evolution
of elytra (called a candidate gene approach). Although effective, candidate
gene lists are finite and can only explore genes that are evolutionarily
maintained across species. In an effort to move fully away from candidate
genes, we used RNA sequencing (RNAseq) to explore the wing transcriptomes of Tribolium during their development. Using this approach, I have revealed
genes enriched in Tribolium elytra (57
genes) as well as genes in the more 'typical' hindwings (29 genes).
Impressively, over a third of the hindwing enriched genes were candidate genes,
which have been analyzed previously in Tribolium.
I also performed a parallel RNAseq analysis in a second beetle species (Dorcus hopei) and compared our results
to Tribolium to reveal conserved
coleopteran elytron and hindwing enriched genes. Although the overlap was
limited, key wing candidate genes were conserved between the two species. Additionally, in both beetles, we discovered
patterns in our RNAseq that suggested that the elytra and hindwing
transcriptomes were highly similar at any given developmental time point, yet
broadly dynamic over even short variations in developmental time. Ultimately, the function of most genes revealed by
RNAseq were unknown (non-candidate genes). To go beyond in
silico-based approaches we used RNA interference-based gene knockdown
(RNAi) to evaluate the function of a subset of these genes in Tribolium. RNAi revealed genes with
roles in forming various aspects of the elytron unique morphology, such as
pigmentation, hardening, sensory structure, and vein formation. In summary, we used
RNAseq and RNAi to reveal and functionally test genes enriched in coleopteran
elytron tissue without relying on Drosophila­­-based
candidate genes. These analyses have provided insight into the formation of a
morphologically novel structure and the mechanisms underlying this critical
evolutionary process.